Interpretive Summary: Acremonium zeae is a type of fungus called an endophyte that is commonly isolated from healthy corn plants or corn kernels showing no visible symptoms of disease. Cultural tests have shown that this endophyte is antagonistic to Aspergillus flavus and Fusarium verticillioides, both species of toxin-producing molds that cause substantial annual losses to the U.S. corn crop. Chemical studies revealed that the metabolites accounting for this activity were two newly reported antibiotics pyrrocidines A and B. There was a need to determine if other important fungal and bacterial pathogens and endophytes of corn are sensitive to pyrrocidines. Pyrrocidine A exhibited potent activity in bioassays against several major stalk and ear rot pathogens of corn. Protective fungal endophytes and mycoparasites which grow asymptomatically within healthy corn tissues, showed little sensitivity to pyrrocidines. Pyrrocidine A also exhibited potent activity against Clavibacter michiganense subsp. Nebraskense, causal agent of Goss’s bacterial wilt of corn, as well as bacteria from corn being evaluated as biocontrol agents. The corn seed industry will benefit from the knowledge that Acremonium zeae can protect corn from virulent pathogens and thus represents a confounding variable impacting the results of corn varietal resistance trials against fungal or bacterial pathogens.

Technical Abstract:
Acremonium zeae produces pyrrocidines A and B, polyketide-amino acid-derived antibiotics, and is recognized as a seed-borne protective endophyte of maize which augments host defenses against microbial pathogens causing seedling blights and stalk rots. Pyrrocidine A displayed significant in vitro activity against Aspergillus flavus and Fusarium verticillioides in assays performed using conidia as inoculum, with pyrrocidine A being more active than B. In equivalent assays performed with conidia and/or hyphal cells as inoculum, pyrrocidine A revealed potent activity against major stalk and ear rot pathogens of maize Fusarium graminearum, Nigrospora oryzae, Stenocarpella (Diplodia) maydis, Rhizoctonia zeae and kernel rotting fungal pathogens A. flavus, Alternaria alternata and Cladosporium cladosporioides. Protective endophytes, including mycoparasites which grow asymptomatically within healthy maize tissues, show little sensitivity to pyrrocidines. Pyrrocidine A also exhibited potent activity against Clavibacter michiganense subsp. Nebraskense, causal agent of Goss’s bacterial wilt of maize, Bacillus mojaviense and Pseudomonas fluorescens, maize endophytes applied as biocontrol agents, but was ineffective against the wilt- producing bacterium, Pantoea stewartii. A. zeae is recognized as a potential confounding variable in maize variety trials for resistance to pathogenic microbes and their mycotoxins.